Hitherto, as a display apparatus which can provide a large video image for allowing the user to feel presence, for example, there are a video projector, an HMD (Head Mount Display), and the like.
However, as shown in FIG. 145, since the video projector optically enlarges a video image and displays it on a screen, not only the video projector but also the screen is necessary to appreciate the video image. In this case, to display a large video image onto the screen, a certain distance is needed between the video projector and the screen. Therefore, if a room is narrow, it is difficult to display a large video image.
On the other hand, an HMD or HUD (Head Up Display) such that a video image to be displayed by a liquid crystal display or the like is enlarged by an optical system such as a lens or the like and a virtual image is formed and this virtual image is provided to the user has been put into practical use in recent years.
In a case where an object exists at a position near the lens than a focal distance, a virtual image is formed on the object side. The details of its forming principle have been disclosed in, for example, Toshifu Ogura, "The Introduction of Science of Lens (the first volume)", Asahi Sonorama Co., Ltd., Kazumi Murata, "Optics", Science Co., Ltd., or the like.
For example, as shown in FIG. 146, the HMD is constructed by including a lens for enlarging a video image and forming a virtual image and a display panel (for instance, liquid crystal display or the like) arranged at a position that is closer than a focal distance of the lens. The user attaches the HMD to a head portion and watches the video image displayed on the display panel through a lens, so that he can appreciate its virtual image. That is, the user can appreciate the large virtual image even if there is no wide space as in case of the video projector.
As shown in FIG. 147, since a width between human eyes (distance between the right and left eyes) is equal to about 56 to 75 mm, as a lens of the HMD, a small lens can be used so long as it can cover such a range. It is known that it is sufficient that the distance from the rotational center of the eyeball to the lens surface is equal to about 35 mm as an average in case of a person with glasses. Therefore, it is possible to construct such that when the HMD is attached to the head portion of the user, the lens is located near the user.
From the above explanation, the HMD can be constructed in a small size and, further, the virtual image is appreciated by using it, so that a large space is not necessary.
A principle such that it is sufficient that the distance from the rotational center of the eyeball to the lens surface is equal to about 35 mm as an average has been disclosed in, for instance, "Glasses Optics", Kyoritsu Publishing Co., Ltd., page 101, or the like (in this literatures it is assumed that a thickness of glasses lens is equal to 8 mm).
However, since the HMD is attached to the head portion and is used, there is a problem that the user feels its attaching sense and a weight.
Although there is a method of using the HMD without attaching it to the head portion, since a condition that the HMD is attached to the head portion and is used is set as a prerequisite, for the purpose of realization of a light weight or the like, a lens diameter is generally set to the necessary minimum size. In the case of using the HMD without attaching to the head portion, accordingly, as shown in FIG. 148, the eyeball is not always located at the front surface (near an optical axis) of the lens and, in many cases, a part of the virtual image is missing and cannot be seen.
Further, the HMD is usually designed so that when it is attached to the head portion, a pupil is located on the optical axis of the lens. A shape of lens is also designed so that when the pupil is located on the optical axis of the lens, aberration becomes the minimum as shown in FIG. 149A. Therefore, in a state where the HMD is used without attaching to the head portion and the pupil is not located on the optical axis of the lens as shown in FIG. 149B, the aberration increases, so that it is difficult to see a clear video image (virtual image).
On the other hand, for example, as shown in FIG. 150, since the HUD is set at a position that is slightly away from the user, a situation that the user feels an attaching sense or a weight as in case of the HMD does not occur.
In the HUD of FIG. 150, the video image displayed on the display panel is enlarged through a lens, the enlarged image is reflected by a half mirror, and the user looks at its reflected light, so that a virtual image is formed. Since the half mirror can transmit the external light, the user can also see an ambient background (situation) as light from the outside which transmits the half mirror together with the virtual image.
The HUD is not used to monitor the video image but is used to observe necessary information while performing some works such as driving of an automobile, control of an airplane, or the like. As mentioned above, therefore, the HUD has been designed so that the user can see an ambient situation, thereby enabling the user to confirm information by a virtual image while concentrating to the work so as not to cause a trouble in the work.
An angle of visibility of a virtual image which is formed by the HUD is set to a narrow angle shown in, for example, FIG. 150 so that the ambient situation can be clearly confirmed.
Therefore, in a case where a video image is appreciated by the HUD, the video image is very hard to see and is lacking in power.
Further, in the HUD, since the position of the virtual image from the user is fixed to about tens of meters ahead in case of the HUD for an automobile, the infinite point in case of the HUD for an airplane, or the like, the virtual image cannot be formed at a desired position of the user.
Since the user hardly moves the head portion during the operation of the automobile or the control of the airplane, in the HUD, the virtual image is formed so that it can be seen only from a predetermined position. It is, consequently, difficult for the user to see the virtual image in a relaxed state while moving the head portion to a certain extent.
Moreover, the HUD is designed such that it is installed at a position that is slightly away from the user (for example, an upper portion of a panel of an automobile, airplane, or the like) so as not to become an obstacle of the work such as operation of the automobile, control of the airplane, or the like as a prerequisite. That is, in order to perform some work by the human being, at least a space where an arm can enter is necessary and the HUD is installed so that such a space can be assured. Therefore, at least the space where the arm can enter is needed between the HUD and the user.
For example, a fact that a front forearm (distance from an arm chin rear edge to a finger tip point when an upper arm is naturally dropped downward and a palm is directed to the inside and the forearm is horizontally bent ahead) of an adult man is equal to 45.1 cm has been disclosed in Jiro Ohara, Ken Uchida, Yoshiyuki Ueno, and Kazutoshi Uchida, "The Human Body is Measured", Nihon shuppan Service Co., Ltd. According to this literature, a wider space is needed between the HUD and the user.
Besides the foregoing HMD and HUD, the virtual image can be also observed by, for example, a view finder or the like of a video camera as shown in FIG. 151. In this case, however, it is necessary to grasp the video camera with the hand or the like and this causes the user to feel a troublesomeness. Even if the video camera is fixed by a tripod or the like, in the viewfinder, since the virtual image can be seen by only one eye, it is hard to obtain a video image with presence.
The invention is made in consideration of such a situation and enables the user to appreciate a virtual image with presence in a relaxed state.